1. Field of the Invention
The present invention generally relates to a print head or drop generator for use with ink jet printers and in particular, to the type of ink jet printers where minute streams of ink are continuously extruded from minute openings in the drop generator.
2. Prior Art
The use of nonimpact printers using multinozzle or single nozzle drop generators for printing readable data on a recording surface is well known in the prior art. Such printers may be divided into the drop-on-demand type printers and the continuous type printers. In the drop-on-demand type printers, a drop of print fluid is generated from the drop generator when needed. In the continuous type printers, continuous streams of ink are extruded from the drop generators. A vibrating crystal vibrates the ink so that the continuous streams are broken up into regularly spaced constant size droplets. The droplets are used for printing on the recording surface.
The prior art abounds with continuous type ink jet printers. Generally, these printers consist of a fluid chamber in which ink (which may be magnetic or conductive) is forced in under pressure. One or more discharging nozzles are disposed to be in fluidic communication with the pressurized ink. A vibrating member is associated with the fluid chamber and excites the chamber so that fluid emanating from the nozzles are broken up into droplets. The droplets are subsequently influenced by electrical or mechanical means to print data onto a recording surface. U.S. Pat. Nos. 3,848,118 and 3,924,974 are examples of this prior art.
Other types of prior art ink jet printers such as those referenced in the above-referenced application, use a dual cavity drop generator. One cavity called the vibrating cavity, houses the vibrating crystal and the other cavity houses the print fluid and the discharging nozzles. The vibrating cavity is filled with a fluid. The fluid conveys pressure waves from the vibrating crystal into the print fluid.
One of the problems which plagues the prior art is the inability to maintain a bubble-free vibrating cavity about the vibrating crystal. Air is introduced during the initial filling of the cavity or may appear with time as fluid is leaked from said cavity. Even if a hermetically sealed cavity is obtained initially, it is extremely difficult to maintain such a sealed cavity over an extended period of time, since the seals about the cavity tend to deteriorate with time.
The introduction of air or vacuum bubbles into the fluid disturbs the uniformity of pressure perturbation along the longitudinal axis of the piezoelectric crystal driver. This results in nonuniform droplet break-off between the streams in a multinozzle ink jet array head. With nonuniform breakoff, the placement of droplets on the recording medium cannot be controlled. The net result is that the quality of the print is rather poor or nonacceptable.
The break-off uniformity of the drop generator is also affected by thermal cycling. Thermal cycling occurs when the temperature of the drop generator changes, usually in response to a change in ambient temperature. Usually there is a difference in the coefficient of expansion between the fluid in the resonance cavity and the material which forms said cavity. As the temperature changes, a mismatch in volume is created between the volume of liquid and the volume of the cavity. The mismatch enhances the probability of air entering the cavity and affects the break-off uniformity of the streams. To correct for thermal cycling, the drop generator has to be operated in an environmentally controlled surrounding or a volume compensator must be attached to the resonance cavity to ensure satisfactory operation. Needless to say, neither of the solutions are acceptable due to cost and undue restriction on the use of the drop generator.
Another problem associated with the prior art drop generator is that the response time is relatively slow. The response time is the time it takes the drop generator to go from a start-up state at zero pressure to an operational state at a predetermined pressure. Stated another way, the response time is the time it takes the drop generator to go from an off condition until the streams are fully established (that is, ready for printing).